Enclosure
The enclosure I picked was the same type I used for rev 3. It has a divider from front panel to rear panel which divides the enclosure into a left side and a right side. the left side has the power IEC input connector, transformers and power switch. It uses about 1/3 of the total space. The left side is the other 2/3 of the space and contains the input and output jacks, the selector switch, the stepped attenuator and the input relay board. This is where the preamp and power supply PC boards go.
I bought the first one in 2022 for $160 with shipping. I was expecting a flimsy sheet metal box, but the front, back and sides are all extruded, and the top and bottom are machined. No wonder the screws all fit without wiggling anything. The box weighs about 9 lbs and is rigid – it doesn’t flex at all. A fitting home for your preamp. The one I have for this version in 2025 cost the same $160. The enclosure comes completely disassembled, with all of the main parts wrapped in bubble wrap and the screws, power inlet and switch, knobs and feet in a bag. The feet are cheap plastic ones, but everything else is top shelf.
Transformers
The transormers are both toroidal with static shielding. They came from AnTek, and cost $14 for tthe 10V 25W and $20 for the 22V 50W. Shipping not included.
The the 50VA transformer with mounting plate (not included. I found it on AliExpress.) You really need one if you don’t want the large center bolt to go through the bottom of the enclosure. Regardless of the enclosure you choose, don’t let the top of the center bolt touch the top of the enclosure. It will form a one turn secondary, drawing as much current as the outlet can provide until your circuit breaker trips or the primary winding melts, or both.